The present invention relates to power supplies for test instruments and, in particular to zero-crossing detectors therefor.
In high precision electrical test instruments, it is often important to determine the zero crossings of the AC power mains that power the instrument. This information is used for synchronization of the instrument, in particular for AC line noise rejection.
Referring to FIG. 1, a prior art zero-crossing detector 10 includes a pair of input terminals 12, 14 for application of an input signal from the AC mains. A rectifier 16 rectifies the AC input signal and the resulting signal is applied across a voltage divider formed by the resistors 18, 20. The voltage across the resistor 20 is applied across the light emitting diode 22A of an optoelectric coupler 22. When the diode 22A is illuminated, the phototransistor 22B of the optoelectric coupler 22 is activated. Ideally, this results in a square wave output, but a logic buffer 24, or the like, may be used to further square up and/or buffer the output signal.
In practice, it has been found that leading and trailing edges of the square wave may not correspond closely enough to the actual zero crossings of the AC mains. In particular, the error compared to the true zero crossing may exceed 100 microseconds.